Archive for the ‘planetary science’ Category

As the Voyager probes moved through the outer Solar System, they compiled a massive record of discovery. Among the newly found objects and phenomena were a large collection of small moons orbiting Jupiter, Saturn, Uranus, and Neptune. Most of these were beyond the ability of Earth-based hardware to image at the time—we actually had to be there to see them.

Since then, however, improvements in ground-based optics and the existence of the Hubble Space Telescope have enabled us to find a few small bodies that had been missed by the Voyagers, as well as other small objects elsewhere in the Solar System, such as the Kuiper Belt object recently visited by New Horizons. Now, researchers have found a way to use advances in computation to increase what we can do with imaging even further, spotting a tiny new moon at Neptune and possibly spotting another for the first time since Voyager 2 was there.

Finding moons

Given that Neptune has been visited by Voyager 2 and imaged frequently since then, any moons we haven't already spotted are going to be pretty hard to see, presumably because they're some combination of small and/or dim. The simplest way to see them is to increase the exposure time, allowing more opportunity for dim signals to emerge from the noise. This method won't work if there's a bright object nearby, which isn't so much of a problem with the outer planets.

Enlarge/ The craters used for this analysis and their locations. (credit: Dr. A. Parker, Southwest Research Institute)

How often does a big rock drop on our planet from space? As we've gotten a better understanding of the impact that did in the dinosaurs, that knowledge has compelled people to take a serious look at how we might detect and divert asteroids that pose a similar threat of planetary extinction. But something even a tenth of the size of the dinosaur-killer could cause catastrophic damage, as you could easily determine by placing a 15km circle over your favorite metropolitan center.

So, what's the risk of having a collision of that nature? It's actually hard to tell. The easiest way to tell is to look for past impact craters and try to figure out the frequency of these impacts, but the Earth has a habit of erasing evidence. So, instead, a group of scientists figured out a clever way of looking at the Moon, which should have a similar level or risk. And they found that the rate of impacts went up about 300 million years ago.

Erasing history

Some impact craters on Earth are pretty obvious, but erosion and infilling with sediments make others much harder to find. We wouldn't have noticed Chicxulub or the Chesapeake Bay Crater were there if we hadn't stumbled across them for other reasons. As we go back in time, plate tectonics can erase evidence of impacts from the sea floor, as the rock they reside in gets subducted back into the mantle. And then, about 550 million years ago, the Great Unconformity wipes off any evidence of impacts that might have been left on land.

While people around the world were celebrating the arrival of 2019, people at the Johns Hopkins Applied Physics Laboratory in Maryland were hard at work. Billions of miles away, the New Horizons probe was flying past Ultima Thule, a small object in the Kuiper Belt. By Tuesday morning, the hardware had sent back a status report that indicated the flyby went as planned, and New Horizons now has lots of data from Ultima Thule that it will slowly send back to Earth over the coming months.

While we don't yet have any of the data that will tell us details about this relic of the Solar System's formation, images taken during the approach solved one of the mysteries that had arisen as New Horizons closed in. But one of the key questions—is Ultima Thule one object or two?—remains unanswered.

Prior to New Horizons' arrival at Ultima Thule, researchers obtained images as it eclipsed a background star. These suggested the body was oblong, rather than spherical. Yet, as the spacecraft got closer, it failed to detect any significant changes in brightness, as you'd expect if an oblong body was rotating.

As we've gathered more details about the other planets of the Solar System, we've largely managed to explain the geography we've found by drawing analogies to things we're familiar with from Earth. Glaciers and wind-driven erosion produce similar results both here and on Mars, for instance. But further out in the Solar System, the materials involved in the geology change—water ice becomes as hard as rock, and methane and nitrogen freeze—which raises the prospect of some entirely unfamiliar processes.

This week, scientists proposed that some weird terrain found on Pluto could be the product of large fields of nitrogen ice sublimating off into the atmosphere. While this explanation could account for some properties of Pluto's geography, it doesn't explain why the process resulted in a series of parallel ridges.

On the washboard

The strange terrain lies to the northwest of Sputnik Planitia, the heart-shaped plane that dominates the side of Pluto we have the best images of. Called "washboard" or "fluted," the area consists of large numbers of roughly parallel ridges with roughly a kilometer or two separating them. Aside from their appearance and general orientation, these ridges don't seem to have a lot in common. They're discontiguous and don't fill the entire region. They run down slopes and spread across valley floors—in some cases a single ridge will run down a slope and then flatten out. And in several cases, they create a starburst-like pattern on along the walls of craters.

Enlarge/ That lovely blue exterior could be hiding a heart of diamond. (credit: NASA)

Carbon, oxygen, and nitrogen are some of the easiest heavier elements to form through fusion. As a result, they’re common in our Solar System, typically found combined with hydrogen to make ammonia, water, and methane. In the gas and ice giants of the outer Solar System, however, these chemicals are placed under extreme pressures, where chemistry starts to get a bit weird. Do these chemicals survive the crushing interiors of these planets?

One intriguing idea is that methane doesn’t survive. As pressure and temperature increase, methane should start condensing into more complex hydrocarbons. Then, as pressures increase further, calculations indicate the hydrogen and carbon should separate out, leaving pure carbon to sink to the depths of these planets. As a result, it’s been hypothesized that, close to their core, planets like Neptune and Uranus have a layer of pure diamond.

While some evidence supporting this theory has surfaced over the years, it’s been hard to precisely replicate the temperatures and pressures found inside the planets. Now, new work done at the SLAC X-ray laser facility supports the idea that these planets are full of diamonds. But the work indicates the diamonds only form at greater depths than we’d previously thought.

Stars like the Sun brighten over the course of their history, a trend that has significant consequences for the habitability of Earth and other bodies both in our Solar System and beyond. An icy world on the far edge of the habitable zone may turn into a temperate paradise given enough time.

Or, it could go straight to being a Venus-style hell if a new study turns out to be right. The study’s authors tuned a full-planet climate model loose on a planet covered in ice. The find that, under a level of incoming light that’s sufficient to melt the ice, the planet reaches a greenhouse state that would cause it to lose all its water to space and possibly head straight into a runaway greenhouse.

The only thing that saved Earth from a runaway greenhouse is, ironically, the presence of greenhouse gasses in its atmosphere.

One of the most important things we’ve learned from the Kepler mission is that, in many ways, our Solar System isn’t unique. Lots of stars have planets, many have multiple planets, and the list of planets includes many with sizes and densities similar to our eight planets. But there are lots of details of our own planets, like the composition and presence of atmospheres, that are much harder to examine at these distances.

One of the features we haven’t gotten a grip on is the presence of moons. Most of our Solar System’s planets have them, and they seem to form by a variety of mechanisms. We’d expect them to be common in exosolar systems, too, but so far we haven’t yet spotted any.

A new paper, which goes into extensive detail about the calculations needed to look for an exomoon, makes it clear why: we simply don’t have enough observation time to pick one up in most cases. But the paper also suggests there may be an exception, as the data hints at a Neptune-sized exomoon, though the statistics aren’t yet conclusive.